Mitskevich (Torsional-Mode Vibration Systems, Sources of High Intensity Ultrasound, Volume 2, Plenum Publishing, NY, 1969, pp. 135-152) disclosed ultrasonic longitudinal-torsional (L/T) resonators in 1967 where their utility in assisting the metal removal action of twist drills was investigated. Wuchinich patented in 2006 (U.S. Pat. No. 6,984,220) such resonators for surgical use in the dissection of hard tissues. Bromfield (U.S. Pat. No. 8,610,334) and Young (U.S. Pat. No. 8,836,200) have subsequently patented longitudinal-torsional ultrasonic transducers for use in surgical operations.
While L/T resonators have proven very useful in assisting the removal of hard tissues such as cortical bone in neurosurgery and orthopedics, they inherently retain a significant encumbrance to their surgical use. Since such resonators produce twisting motion of the tissue excising tip, they contain a component of motion that is longitudinal. Prior art ultrasonic aspirators, used principally for dissection of soft tissue, possess almost entirely longitudinal motion, that is, motion parallel to the longitudinal axis of their slender tips. While this motion induces cavitation of the tissue's intracellular water, thereby rupturing the cell walls and allowing evacuation of their contents and is consummately effective in dissecting soft tissue from low water content tissue such as blood vessels and fascia, this motion also induces cavitation in the irrigation fluid flowing over the tip. Irrigation is essential to the to the operation of such resonators, as the tip must be kept cool and the excised tissue must be lubricated as it is evacuated through the tip to prevent clogging that would otherwise occur from the desiccating effect of the vibration and suction employed to remove the excised tissue through a bore in the tip. Since the motion of L/T tips contains a longitudinal component, these tips also induce cavitation which, however, is not effective in removing hard tissues. It is rather the abrasive effects of the motion possessed by these tips, usually equipped with a burr that is the principal agent of dissection. The unavoidable cavitation mist produced by the longitudinal motion obscures the visual field, preventing precise dissection by the surgeon whose view of the actual surgical site is obstructed.
Torsional resonators are well known in the art and torsional transducers needed to excite such resonators are described at length by Mitskevich as well. In practice, however, designing torsional ultrasonic transducers to power and excite torsional resonators has proved challenging. They are invariably complicated to construct and difficult to incorporate in an appliance that can be facilely manipulated in micro-surgery. But there remains a need for their use where the absence of a longitudinal component of motion sufficient to produce cavitation and mist which blurs vision is a distinct benefit and enhancement.
L/T resonators, however, are excited by longitudinal transducers whose technology is well known in the art and whose design for all manner of applications, including micro-surgical procedures, has been well refined. If longitudinal transducers could be used to excite overwhelmingly torsional motion in a surgical tip, a major advancement in the ultrasonic excision of hard tissue would be achieved.